• Journal of computational fluids engineering
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• v.20 no.2
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• pp.7-15
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• 2015

This paper attempts to evaluate the accuracy and efficiency of a design optimization procedure by combining wavelets-based multi resolution analysis method and proper orthogonal decomposition (POD) technique. Aerodynamic design procedure calls for high fidelity computational fluid dynamic (CFD) simulations and the consideration of large number of flow conditions and design constraints. Thus, even with significant computing power advancement, current level of integrated design process requires substantial computing time and resources. POD reduces the degree of freedom of full system by conducting singular value decomposition for various field simulations. In this research, POD combined Design Optimization model is proposed and its efficiency and accuracy are to be evaluated. For additional efficiency improvement of the procedure, multi resolution analysis method is also being employed during snapshot constructions (POD training period). The proposed design procedure was applied to the optimization of wing aerodynamic performance. Throughout the research, it was confirmed that the POD/MRA design procedure could significantly reduce the total design turnaround time and also capture all detailed complex flow features as in full order analysis.

• Journal of the Korean Society for Precision Engineering
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• v.32 no.12
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• pp.1065-1072
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• 2015

This paper is about the development of an insole sensor system that can determine the model of an exoskeleton robot for lower limb that is a multi-degree of freedom system. First, the study analyzed the kinematic model of an exoskeleton robot for the lower limb that changes according to the gait phase detection of a human. Based on the ground reaction force (GRF), which is generated when walking, to proceed with insole sensor development, the sensing type, location, and the number of sensors were selected. The center of pressure (COP) of the human foot was understood first, prior to the development of algorithm. Using the COP, an algorithm was developed that is capable of detecting the gait phase with small number of sensors. An experiment at 3 km/h speed was conducted on the developed sensor system to evaluate the developed insole sensor system and the gait phase detection algorithm.

• Asia pacific journal of information systems
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• v.20 no.1
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• pp.57-79
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• 2010

In a virtual community, one can possess multiple identities and pretend to be different by creating self-identity in contrast with his or her actual self. Does false identity undermine the qualitative growth of a virtual community by reducing members' accountability? Or does it stimulate their contributive behaviors by ensuring freedom of speech? It is imperative to understand the effects of multi-identity considering the distinct properties of a virtual community in which people easily change their identities at little or no cost. To answer these questions, we adopted the concept of self-discrepancy from the social psychology theory rooted in the concept of the self and developed a theoretical model to predict quality of contribution of the individual member in virtual communities. Based on the self-discrepancy theory, we first identified two different domains of the self: (1) an "actual self" that consists of attributes that the person believes he or she currently possesses in real life and (2) a "cyber self" that consists of attributes the person believes he or she possesses in a virtual community. Next, we derived an index for two different types of self-discrepancy by using the differences between the actual and the cyber identities: Personal Self-discrepancy and Social Self-discrepancy. Personal Self-discrepancy reflects the degree of discrepancy between actual and cyber identity regarding a person's intelligence, education, and expertise. Social Self-discrepancy reflects the degree of discrepancy between actual and cyber identity regarding a person's morality, sociability, and accordance with social norms. Finally, we linked them with sense of virtual community, perceived privacy rights, and quality of contribution to examine how having a multi-identity influences an individual's psychological state and contributive behaviors in a virtual community. The results of the analysis based on 266 respondents showed that Social Self-discrepancy negatively influenced both the Sense of Virtual Community and Perceived Privacy Rights, while Personal Self-discrepancy negatively influenced only Perceived Privacy Rights, thereby resulting in reduced quality of contribution in virtual communities. Based on the results of this analysis, we can explain the dysfunctions of multi-identity in virtual communities. First, people who pretend to be different by engaging in socially undesirable behaviors under their alternative identities are more likely to suffer lower levels of psychological wellbeing and thus experience lower levels of sense of virtual community than others. Second, people do not perceive a high level of privacy rights reflecting catharsis, recovery, or autonomy, even though they create different selves and engage in socially undesirable behaviors in a virtual community. Third, people who pretend to be different persons in terms of their intelligence, education, or expertise also indirectly debase the quality of contribution by decreasing perceived privacy rights. The results suggest that virtual community managers should pay more attention to the negative influences exercised by multi-identity on the quality of contribution, thereby controlling the need to create alternative identities in virtual communities. We hope that more research will be conducted on this underexplored area of multi-identity and that our theoretical framework will serve as a useful conceptual tool for all endeavors.

• Smart Structures and Systems
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• v.33 no.2
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• pp.145-163
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• 2024

The evaluation of dynamic characteristics of bridges under operational traffic loads is a crucial aspect of bridge structural health monitoring. In the vehicle-bridge interaction (VBI) system, the vibration responses of bridge exhibit time-varying characteristics. To address this issue, an accurate time-frequency analysis method that combines the autoregressive power spectrum based empirical wavelet transform (AR-EWT) and local maximum synchrosqueezing transform (LMSST) is proposed to identify the time-varying instantaneous frequencies (IFs) of the bridge in the VBI system. The AR-EWT method decomposes the vibration response of the bridge into mono-component signals. Then, LMSST is employed to identify the IFs of each mono-component signal. The AR-EWT combined with the LMSST method (AR-EWT+LMSST) can resolve the problem that LMSST cannot effectively identify the multi-component signals with weak amplitude components. The proposed AR-EWT+LMSST method is compared with some advanced time-frequency analysis techniques such as synchrosqueezing transform (SST), synchroextracting transform (SET), and LMSST. The results demonstrate that the proposed AR-EWT+LMSST method can improve the accuracy of identified IFs. The effectiveness and applicability of the proposed method are validated through a multi-component signal, a VBI numerical model with a four-degree-of-freedom half-car, and a VBI model experiment. The effect of vehicle characteristics, vehicle speed, and road surface roughness on the identified IFs of bridge are investigated.

• Journal of Korean Association for Spatial Structures
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• v.11 no.2
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• pp.101-110
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• 2011

Many researchers have attempted to apply semi-active control systems in the civil engineering structures. Recently, magneto-rheological(MR) fluid dampers have been developed. This MR damper is one of semi-active dampers as a new class of smart dampers. This paper discusses the application of MR damper for seismic response control of adjacent buildings subjected to earthquake. Here, a controllable damping force of MR damper that is installed between adjacent buildings is applied to seismic response control. A hybrid model combines skyhook and groundhook control algorithm so that the benefits of each can be combined together. In this paper, hybrid control model are applied to the multi degree of freedom system representative of buildings in order to reduce seismic response of adjacent buildings. And the performance of hybrid control model is compared with that of others. It was demonstrated that hybrid control model or adjacent buildings with MR damper was effective for seismic response control of two adjacent buildings reciprocally.

• International Journal of Naval Architecture and Ocean Engineering
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• v.13 no.1
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• pp.746-757
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• 2021

To investigate the mechanism of friction-induced vibration and noise of ship water lubricated stern bearings, a two-degree-of-freedom (2-DOF) nonlinear self-excited vibration model is established. The novelty of this work lies in the detailed analysis of influence of different parameters on the stability and nonlinear vibration characteristics of the system, which provides a theoretical basis for the various friction vibration and noise phenomenon and has a very important directive meaning for low noise design of water lubricated stern bearings. The results reveal that the change of any parameter, such as rotating speed of shaft, contact pressure, friction coefficient, system damping and stiffness, has an important influence on the stability and nonlinear response of the system. The vibration amplitudes of the system increase as (a) rotating speed of shaft, contact pressure, and the ratio of static friction coefficient to dynamic friction coefficient increase and (b) the transmission damping between motor and shaft decreases. The frequency spectrum of the system is modulated by the first mode natural frequency, which is continuous multi-harmonics of the first mode natural frequency. The response of the system presents a quasi-periodic motion.

• Structural Engineering and Mechanics
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• v.55 no.6
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• pp.1241-1260
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• 2015

Any rational approach to define the configuration and size of viscous fluid dampers in a structure should be based on the dynamic properties of the system with the dampers. In this paper we propose an alternative representation of the complex eigenvalues of multi degree of freedom systems with dampers to calculate new equivalent natural frequencies. Analytical expressions for the dynamic properties of a two-story building model with a linear viscous damper in the first floor (i.e. with a non-proportional damping matrix) are derived. The formulas permit to obtain the equivalent damping ratios and equivalent natural frequencies for all the modes as a function of the mass, stiffness and damping coefficient for underdamped and overdamped systems. It is shown that the commonly used formula to define the equivalent natural frequency is not applicable for this type of system and for others where the damping matrix is not proportional to the mass matrix, stiffness matrix or both. Moreover, the new expressions for the equivalent natural frequencies expose a novel phenomenon; the use of viscous fluid dampers can modify the vibration frequencies of the structure. The significance of the new equivalent natural frequencies is expounded by means of a simulated free vibration test. The proposed approach may offer a new perspective to study the effect of viscous dampers on the dynamic properties of a structure.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.11
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• pp.1765-1772
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• 1997

The layered composites have a character to change of structure stiffness with respect to the layup angles. The deformations in the fan-blades to be initially designed by considering efficiency and noise, etc., which arise due to the pressure during the fan operation, can make the fan inefficient. Thus, so as to minimize the deformations of the blades, it is needed to increase the stiffness of the blades. An investigation has been performed to develop the three dimensional layered composite shell element with the drilling degree of freedom and the optimization module for finding optimal layup angles with sensitivity analysis. And then they have been verified. In this study, the analysis model is engine cooling fan of automobile. In order to analyzes the stiffness of the composite fan blades, finite element analysis is performed. In addition, it is linked with optimal design process, and then the optimal angles that can maximize the stiffness of the blades are found. In the optimal design process, the deformations of the blades are considered as multiobjective functions, and this results minimum bending and twisting simultaneously.

• Earthquakes and Structures
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• v.18 no.1
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• pp.45-56
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• 2020

The application of critical excitation method with displacement-based objective function for multi degree of freedom (MDOF) systems is investigated. To this end, a new critical excitation method is developed to find the critical input motion of a MDOF system as a synthetic accelerogram. The upper bound of earthquake input energy per unit mass is considered as a new constraint for the problem, and its advantages are discussed. Considering this constraint, the critical excitation method is then used to generate synthetic accelerograms for MDOF models corresponding to three shear buildings of 10, 16, and 22 stories. In order to demonstrate the reliability of generated accelerograms to estimate dynamic response of the structures, three target ground motions with considerable level of energy contents are selected to represent "real critical excitation" of each model, and the method is used to re-generate these ground motions. Afterwards, linear dynamic analyses are conducted using these accelerograms along with the generated critical excitations, to investigate the key parameters of response including maximum displacement, maximum interstory drift, and maximum absolute acceleration of stories. The results show that the generated critical excitations can make an acceptable estimate of the structural behavior compared to the target ground motions. Therefore, the method can be reliably implemented to generate critical excitation of the structure when real one is not available.

• Journal of Institute of Control, Robotics and Systems
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• v.15 no.5
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• pp.543-547
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• 2009

This paper describes the design, simulation, development, and experiment of a six degree-of-freedom micropositioning parallel manipulator. A movable stage was supported with six links, each of which extends with a dc-servo micropositioning actuator. In case of parallel manipulator, while the solution of the inverse kinematics is easily found by the vectors of the links which are composed of the joint coordinates in base and platform, but forward kinematic is not easily solved because of the nonlinearity and complexity of the parallel manipulator's kinematic output equation with the multi-solutions. The movable range of the prototype was ${\pm}25mm$ in the x- and y-directions and ${\pm}12.5mm$ in the z-direction. The minimum incremental motion of the prototype was $1{\mu}m$ in the x- and y-directions and $0.5{\mu}m$ in the z-direction. The repeatability of the prototype was ${\pm}2{\mu}m$ in the x- and y-directions and ${\pm}1{\mu}m$ in the z-direction. The motion performance was also evaluated by not only the computer simulation of CAD model but also the experiment using a capacitive sensor system.